RLL is a modulation technique used by hard disk drives to code binary data (0s and 1s) into magnetic information. It replaced the old MFM modulation.
Instead of modulation one bit at a time, as it occurs on FM and MFM methods, this technique codes several bits per time. In order to work this modulation requires two variables: run length, which is the minimum space between signal inversions, and run limit, which is the maximum space between signal inversions, where ”run“ refers to the space sequence on the modulated data without signal inversion.
RLL modulation is referred like RLL(x,y) where x is the run length and y is the run limit. For example, at RLL(2,7), the minimum space between signal inversions is of two spaces and the maximum space between signal inversions is of seven spaces.
RLL(2,7) modulation makes data to use 2/3 of the physical space compared to MFM modulation and 1/3 of the physical space compared to FM modulation. So this modulation technique is more efficient than the old MFM modulation, as it allows more data to be stored using the same physical space.
Another very common RLL modulation standard used on hard disk drives is RLL(3,9), which is also known as ARLL (Advanced RLL). Under this modulation data can be stored using half the physical space compared to MFM.